The Effects of Boric Acid and Phosphoric Acid on The

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  Theeffectsofboricacidandphosphoricacidonthecompressive strength of glass-ionomer cements Leon H. Prentice, Martin J. Tyas * , Michael F. Burrow School of Dental Science, University of Melbourne, 711 Elizabeth Street, Parkville, Victoria 3010, Australia Received 25 November 2004; accepted 7 April 2005 KEYWORDS Dental material;Glass-ionomer;Phosphoric acid;Boric acid;Compressive strength;Fluoride glass;Polyalkenoate cement Summary  Objectives : Both boric acid (H 3 BO 3 ) and phosphoric acid (H 3 PO 4 ) arecomponents of dental cements, commonly incorporated into glass (as ingredients inthe melt) and occasionally added to the powder or liquid components. This studyinvestigated the effect of boric acid addition to an experimental glass-ionomerpowder and the effect of phosphoric acid addition to a glass-ionomer liquid on the24-h compressive strength. Methods : Boric acid powder was added in various concentrations to an experimentalglass-ionomer powder and, separately, phosphoric acid was added to anexperimental glass-ionomer liquid. Powders and liquids were dosed into capsulesat various powder:liquid ratios and cements thus formed were assessed for 24-hcompressive strength. Results : Incorporation of boric acid in glass-ionomer powder resulted in apronounced decrease (  p ! 0.05 at 1% boric acid) in compressive strength. Additionof phosphoric acid produced initially stronger cements (up to 13% increase at 1%phosphoric acid) before also declining. Significance : The incorporation of less than 2% w/w phosphoric acid in glass-ionomerliquids may improve cement strengths without compromising clinical usefulness. Theincorporation of boric acid in glass-ionomer cements is contraindicated. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. Introduction Glass-ionomer restorative cements derive theirchemistry from silicate cements and zinc polyalk-enoates [1], by combining the silica glasses ofthe first with polyalkenoic acids from the second.Both boron salts and phosphates are commoningredients in silicate cements [2,3], and phos-phates especially are well-known in glass-ionomerglasses [4–6]. Previous work on borax (hydratedsodium borate: Na 2 B 4 O 7 .10H 2 O) as a powder addi-tive has suggested its usefulness [7], but nosignificant improvements in viscosity, settingcharacteristics, or strength were noted. Very littleinformation has been published on the roles ofeither phosphoric acid in glass-ionomer liquid orboric acid as an additive to the powder. The general Dental Materials (2006)  22 , 94–$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.doi:10.1016/ *  Corresponding author. Tel.:  C 61 3 9341 0231; fax:  C 61 39341 0437. E-mail address: (M.J. Tyas).  setting reaction of a glass-ionomer cement,whereby divalent cations cause the early cross-linking of the polyalkenoic acids and trivalent(normally aluminium) cations promote maturation,has been widely investigated [8,9]. The role ofother species remains uncertain, in particular therole of tartaric acid and other acids.The aim of this investigation was to examine theeffects of boric acid addition to a glass-ionomerpowder and, separately, phosphoric acid additionto a glass-ionomer liquid. The null hypothesis is thatsuch additions have no effect on the 24-h compres-sive strength of an experimental glass-ionomercement. Materials and methods Powder and liquid preparation Experimental glass-ionomer powder and liquid weresupplied by SDI Ltd, Bayswater, Australia. Thepowder contained strontium fluoroaluminosilicateglass and a powdered polyalkenoic acid; the liquidcontained a polyalkenoic acid, tartaric acid, andwater. Boric acid ( O 99%, Sigma-Aldrich Ltd,Sydney, Australia) was ground and sieved througha 150- m m mesh followed by incorporation into theexperimental powder (SDI Ltd) at concentrations of1, 2, 3 and 7% by weight. Unblended powder wasused as a control. Four capsules (Riva SC, SDI Ltd)were dosed for each powder, at powder:liquidratios of 3:1 and 3.55:1, for a total of 40 capsules.Three specimens were prepared for compressivestrength testing from each capsule.Separately, 81% orthophosphoric acid (DeltrexChemicals, Melbourne, Australia) was incorporatedinto an experimental liquid (SDI Ltd) to producephosphoric acid concentrations of 1, 2, 3 and 7% byweight. Four capsules were dosed for each liquid atpowder:liquid ratios of 3:1 and 3.4:1, following thepilot observation that powder:liquid ratios above3.4:1 resulted in pastes too viscous for adequateevaluation, for a total of 40 capsules. Again, threespecimens were prepared for compressive strengthevaluation from each capsule. Compressive strength Compressive strength was evaluated according toISO9917 [10]. Capsules were activated and mixed ina high-speed mixer (Ultramat 2, SDI Ltd) at (4500 G 100) Hz for 10 s. Cylindrical specimens of diameter(4.0 G 0.1) mm and height (6.0 G 0.1) mm wereprepared in stainless steel split moulds and kept for1 h at 37  8 C in an environment of  O 80% relativehumidity, removed from the moulds and immersedin distilled water at 37  8 C. After 23 h, specimenswere removed from the water, sanded on 800-gritSiC paper toensureparallelends,andloaded axiallyto fracture on a universal testing machine (Instron5566, Instron Ltd, Milton Keynes, UK).Data were analyzed with one-way analysis ofvariance and pairwise  t -tests, using a significancevalue of  p Z 0.05. Results Compressive strength Incorporation of boric acid resulted in a significantreduction in compressive strength of the glass-ionomer cement (  p ! 0.05). Cements with 1% boricacid in the powder exhibited compressive strengthsof 88.7 and 95.4 MPa at powder:liquid ratios of 3:1and 3.55:1, respectively (Table 1), which wassignificantly lower than the control (105.1 and119.8 MPa, respectively). Concentrations of 2, 4,and8%w/wboricacidwereweakeragain(Figure1);addition of 8% boric acid resulted in cements with24-h compressive strengths of only 42.6 and46.6 MPa at powder:liquid ratios of 3:1 and3.55:1, respectively.Compressive strengths for cements incorporatingphosphoric acid were greater with liquids incorpor-ating 1 and 2% phosphoric acid, but reduced as theconcentration of phosphoric acid increased(Figure 2). Strength maxima at 1% phosphoric acidwere 119.4 MPa and 123.5 MPa at powder:liquidratios of 3:1 and 3.4:1, respectively (Table 2). Table 1  Compressive strength of glass-ionomercements with boric acid incorporated in powder attwo powder:liquid ratios.Compressive strength, MPa% Boric acidin powderPowder:liquid ratio3:1 3.55:10 105.1 (0.3) a 119.8 (8.4) a 1 88.7 (1.3) b 95.4 (1.9) b 2 74.5 (2.7) c 74.4 (4.3) c 4 65.5 (0.4) d 61 (1.2) d 8 46.6 (2.1) e 42.6 (1.1) e Numbers in brackets are SD; superscripts denote significantdifferences (  p ! 0.05) within columns.  n Z 12 for each group. Boric and phosphoric acid additives to GICs 95  Discussion Boric acid The use of boric acid as a reaction modifier inphosphoric acid-containing industrial cements,which have some chemical similarities to glass-ionomers, has been previously noted [11]. In thesesystems, boric acid at 3–4% by weight in the finalcement contributed to a decrease in the rate ofreactivity, without significant loss of strength.Water uptake of solid boric acid may increasereactivity by reducing the water content availablefor flowability and ion transfer, reducing the degreeoffinal cross-linking of the polyalkenoate species inthe set cement. Boric acid, as HBO 32 K or BO 33 K , mayact as a weak polyalkenoate cross-linker [12], butthis effect is weaker and slower than for metal ions,so it is likely that boric acid interferes with, ratherthan aids, the acid–base glass-ionomer reaction.Both boric acid and phosphoric acid are triproticacids, and can be expected to undergo covalentbonding to some extent with the various cationicspecies released from the glass-ionomer glassduring initial acid attack. Boric acid is a very weakacid (pK a Z 9.14), which indicates that at the acidicpH conditions in glass-ionomer liquids (typicallypH z 2), boric acid will most likely remain fullyprotonated, and hence unreactive. In fact, boricacid takes up water and can be viewed chemicallyas boron oxide trihydrate, which is only slightlyreactive. The boric acid particles may remain onlypartially reacted in situ, resulting in weakeningthrough dissolution effects in the hydrated matrix.This lack of reactivity and strength reiterated theunique strengthening and rheology modification oftartaric acid [9,13,14]. Phosphoric acid The initial increase in strength of the cementcaused by phosphoric acid may be due primarily tothe role of phosphoric acid in cross-linking thenetwork. Phosphate in the dentin or enamelstructure may contribute to bonding with toothstructure [15,16]; in a similar manner the phospho-ric acid in the matrix may contribute to cross-linkingof the carboxylic acid groups, or, as phosphate,precipitate into the glass-ionomer matrix [17].Previous work has demonstrated the efficacy ofcalcium phosphate incorporation as an amorphoussolid in the powder component [18], though anassessment of the existence and distribution ofcalcium or strontium phosphates in the final, setcement may be the topic for a subsequent study.The low dissociationconstantforphosphoric acid(pK a Z 2.12) ensures its reactivity in the glass-ionomer system. Improvements in strength with 1or 2% w/w phosphoric acid addition to the liquidmay be due to its ability to cross-link the matrix,and act as a matrix-former itself [19]. Severalstudies have analyzed the role of phosphoric acidin analogous cement formation and in bonding[20–22], though none has focused on the role of 020406080100120140 Percentage H 3 PO 4  in liquid    C  o  m  p  r  e  s  s   i  v  e   S   t  r  e  n  g   t   h   (   M   P  a   ) P:L ratio 3:1P:L ratio 3.4:1 10987 6543210 Figure 2  Compressive strength of glass-ionomercements with phosphoric acid incorporated in liquid atpowder:liquid ratios of 3:1 and 3.4:1. 02040608010012014010987 6543210 Percentage H 3 BO 3  in powder    C  o  m  p  r  e  s  s   i  v  e   S   t  r  e  n  g   t   h   (   M   P  a   ) P:L ratio 3:1P:L ratio 3.55:1 Figure 1  Compressive strength of glass-ionomercements with boric acid incorporated in powder atpowder:liquid ratios of 3:1 and 3.55:1. Table 2  Compressive strength of glass-ionomercements with phosphoric acid incorporated in liquidat two powder:liquid ratios.Compressive strength, MPa% Phosphoric acidin liquidPowder:liquid ratio3:1 3.4:10 105.1 (0.3) b 119.8 (8.4) a 1 119.4 (1.1) a 123.5 (6.9) a 2 118.7 (2.2) a 117.1 (2.7) a 3 100.9 (1.6) c 102.5 (2.8) b 7 48.2 (2.8) d Unable to formcement Numbers in brackets are SD; superscripts denote significantdifferences (  p ! 0.05) within columns.  n Z 12 for each group. L.H. Prentice et al.96  phosphoric acid in the acid–base reaction or setcement. Phosphoric acid has been used previouslywith metal oxides to form cements (e.g. zincphosphate cement), and a similar cross-linkingeffect with the strontium and aluminium ions mayhave caused the significant strength increase foundin the present study. Further work, in particularFourier-Transform Infra-Red Spectroscopy orNuclear Magnetic Resonance analysis of the role ofphosphoric acid, would further elucidate its poten-tial role in strengthening glass-ionomer cements. Conclusion The null hypothesis was rejected. The addition of 1to 2% w/w phosphoric acid to glass-ionomer liquidsmay improve compressive strength of the cementswhile maintaining clinically useful cement proper-ties. The addition of boric acid to the glass-ionomercement powder resulted in significantly decreasedstrength, and is contra-indicated. Acknowledgements The authors wish to thank SDI Ltd for the supply ofmaterials for this study. References [1] WilsonAD,KentBE.Anewtranslucentcementfordentistry.The glass ionomer cement.  Br Dent J  1972; 132 :133–5.[2] Wilson AD. Dental silicate cements: VII. Alternative liquidcement formers.  J Dent Res  1968; 47 :1133–6.[3] Wilson AD, Kent BE, Batchelor RF. Dental silicate cements.IV. 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Topical fluorideand glass ionomer microhardness.  Am J Dent  1995; 8 :134–6.[22] Yoshida Y, van Meerbeek B, Nakayama Y, Snauwaert J,Hellemans L, Lambrechts P, et al. Evidence of chemicalbonding at biomaterial-hard tissue interfaces.  J Dent Res 2000; 79 :709–14. Boric and phosphoric acid additives to GICs 97

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